The Origin and Evolution of Mammals - Moodle

4 THE ORIGIN AND EVOLUTION OF MAMMALS
levels varied, and ice ages alternated with warm
times. Thus, for much of the Tertiary, independent
evolutionary radiations of mammals were occurring
simultaneously in different areas, with secondary
contacts between hitherto isolated faunas adding
complex patterns of dispersal and competition. As
time passed the mammalian fauna contained more
and more of the familiar groups of today. The final
great event in the evolution of mammals, and the
cause of the last touches to the shaping of the modern
fauna, occurred a mere 10–20 thousand years
ago, when most of the species of larger mammals
disappeared from the fossil record. Whether this
was due to environmental change or over-hunting
by humans is still vigorously debated.
Palaeobiological questions
The quality of the fossil record documenting this
series of events is remarkably good. Vertebrate
skeletons generally have a high capacity for being
preserved, and because they are so anatomically
complex, many characters are potentially available
for taxonomic analysis. A further consequence of its
complex, multifunctional nature is that a more or
less complete, well-preserved vertebrate skeleton
reveals a great deal of biological information about
the life of its one-time possessor. Unfortunately, set
against this high information content is the fact that
good specimens do tend to be rare, because vertebrates
are relatively large animals that therefore
live in relatively small populations compared to
many other kinds of organisms. In the case of terrestrial
vertebrates the rarity is exaggerated because
the only usual way that a fossil forms is if the dead
organism suffers the unlikely fate of ending up
buried in sediments forming on the bed of a lake,
delta, or sea. There are wonderful exceptions to this
rule of improbability, and a handful of freakish conditions
were responsible for some Lagerstätten in
which the preservation of mammals and other taxa
is superb, and which open unique windows onto
the faunas of the time. The Early Cretaceous Yixian
Formation of Liaoning Province of China is one
such (Luo 1999; Zhou et al. 2003), with complete
skeletons of a variety of little mammals, some even
with impressions of the pelt. The Grube Messel
Eocene locality, an old opencast oil-shale mine near
Frankfurt that is early Eocene in age contains a
sample of forest-dwelling mammals of the time that
even includes gut contents and muscle and skin
impressions (Schaal and Zeigler 1992). At the other
end of the temporal scale, the famous Rancha La
Brea asphalt pits, within the city boundary of Los
Angeles, were trapping mammals in seeping oil
from the Late Pleistocene 40,000 years ago to about
4,500 years ago (Marcus and Berger 1984). Even
such fossiliferous localities of these do not break the
general rule that fossil tetrapod vertebrates are
individually highly informative, but collectively
very uncommon.
The palaeobiological questions that can be
addressed with reference to the origin and evolution
of mammals are circumscribed by these qualities
of the synapsid fossil record. Microevolutionary
issues such as intraspecific changes in response to
selection pressures, or the process of speciation cannot
usually be approached using these particular
fossils, although there are a number of cases where
Tertiary mammal species have been used to demonstrate
microevolutionary trends, particularly,
though not exclusively from the Pleistocene, as any
palaeobiological textbook will attest. However,
there are several general macroevolutionary questions
concerning events whose time course is a matter
of millions to tens and hundreds of millions of
years where the synapsid fossil record has been and
continues to be extremely important.
Concerning the first phase of synapsid radiation,
the big question is that of how a major new kind of
organism evolves, and the case of the origin of
mammals is far and away the example with the
most revealing fossil record. Very many characters
changed in the course of the transition from ancestral
amniote to mammal, and yet biological integration
between all the structures and processes of
each and every intermediate organism must have
been maintained. From the pattern and sequence of
acquisition of mammalian characters inferred from
the cladistic analysis of mammal-like reptiles, much
light is thrown on the process of the origin of a new
higher taxon. From palaeoecological analysis,
something of the environmental backdrop to the
process is revealed.
The most compelling macroevolutionary question
concerning the second phase of synapsid evolution,